1. Field of the Invention
The present invention relates to the combustion box of a gas furnace. More particularly, the field of the invention is that of combustion box construction for gas furnaces utilizing oscillating combustion.
2. Description of the Related Art
Gas furnaces typically have gas burners mounted within a combustion box which provide heated products of combustion to heat exchangers disposed within the plenum of a furnace. The heating function of the furnace is accomplished by circuiting indoor air over these heat exchangers to warm the air prior to its exit from the furnace. A significant problem with many gas furnaces pertains to the occurrence within the combustion box of oscillating combustion, which often generates undesirable acoustical noise and vibration. More specifically, during the operation of gas burners within a combustion box, generation of the products of combustion creates pressure oscillations and an oscillating flame which cause acoustical noise and vibration within the combustion box. Without any compensation for the oscillating combustion, the noise and vibrations generated pass into the heat exchangers, from where they are transmitted to the passing indoor air. This transmission may result in the undesirable noise and vibration penetrating the interior of the building being heated, and causing discomfort to the individuals within the building.
Prior art structures are known which attempt to dampen the noise and vibration resulting from oscillating combustion. These prior art structures involve various dampening or absorbing devices disposed in the combustion box on a wall which oscillates during pressure oscillations in the box. For example, as disclosed in U.S. Pat. No. 4,411,616, an oscillatable wall is disposed in a combustion chamber, such as a flexurally soft thin sheet metal plate backed by a vibration absorbing mass or a rigid plate backed by a sound absorber, to avoid the excitation of combustion chamber oscillations. The flexurally soft wall of a combustion box is backed by a vibration absorbing composition such as a mixture of rubber, tar products, and plastics while the rigid plate is backed by a hydraulic dashpot and mounted by a bellows structure of sheet metal. Other arrangements of vibration absorbing or dampening devices are also known for similar purposes. U.S. Pat. No. 2,224,130 teaches a construction which accounts for pressure oscillations by placing a loosely fitted flexible fabric over a hole of the combustion box so that pressure oscillations are reduced by the movement of the fabric, which changes the effective volume of the combustion chamber. While these prior art devices reduce the effects of oscillating combustion within the combustion chamber, noise and vibration generated within the combustion box which is not absorbed or dampened is communicated into the heat exchangers. Such noise and vibration tends to pass into the indoor air ducts and ultimately into the space being heated. Therefore, it is desirable to provide a combustion chamber wherein noise is removed, communicated, or otherwise dissipated therefrom, without being passed into the heat exchangers.